The presence of triiodide, I3–, throughout the lifetime of a persulfate–iodide reaction with persulfate in excess, is controlled by an opposing effect of falling iodide concentration and rising iodine concentration. This brings triiodide to a concentration maximum, [I3–]max, which can be studied by UV–visible spectroscopy using absorbance measurements taken over an extended iodine clock time-scan. Equilibrium and conservation equations can be combined so that, together with [I3–]max, a value of the constant K for the equilibrium established between iodine, iodide, and the triiodide complex ion can be obtained.
Burgess, A. E., & Davidson, J. C. (2012). A kinetic–equilibrium study of a triiodide concentration maximum formed by the persulfate–iodide reaction. Journal of Chemical Education, 89(6), 814–816. https://doi.org/10.1021/ed200055t